The present invention relates to plural-layer membranes generally, and to optical membranes or pellicles that are used in the semi-conductor chip industry. More particularly, the present invention relates to a method of forming a finished edge on plural-layer optical membranes that is used in connection with the manufacture of pellicles.
In the semi-conductor chip industry it is well known that pattern transfer from the photo mask to substrate is accomplished by directing light from a suitable light source through the mask to the substrate. During the pattern transfer process, also called the photolithographic process, the patterns on the photo mask are projected onto the substrate which has been treated with a photo-sensitive substance. This results in the mask pattern being reproduced on to the substrate.
A manufacturing problem is encountered when foreign substances are present on the surface of the mask. There is a problem because foreign substances on the mask surface will also be reproduced on the substrate and therefore will interfere with proper pattern transfer.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on it. The pellicle membrane extends parallel to the mask, and at a predetermined distance spaced away from the mask. Any contamination which would ordinarily land on the mask surface, falls onto the pellicle membrane rather than onto the mask.
Pellicles eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or the pellicle frame. Such particles may drop onto the mask surface during photolithography which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely uniform across its surface. Membrane uniformity ensures that light passing through the membrane during photolithography will not be obstructed or refracted in any way. The composition of the membrane must be highly uniform, and the membrane must be tensioned evenly across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the relatively thin membrane and the frame.
To further understand these important requirements, it is necessary to provide an explanation of how pellicles are formed. For purposes of the present invention, conventional plural-layer pellicle fabrication will be discussed.
As is known in the art, forming a plural layer optical membrane is the first step in plural-layer pellicle manufacture. Commonly a first, or core, layer is formed by spinning a suitable polymer, such as nitrocellulose-based one, on a substrate. Next, additional so-called antireflective (AR-) coating layers are formed on opposing sides of the core layer using conventional processes such as spinning. Commonly, a five-layer optical membrane is formed with a core layer, single opposing intermediate AR-coating layers, and single opposing outer AR-coating layers.
After the plural layer membrane is formed it is held under tension adjacent its periphery by an outer frame to prepare it for subsequent manufacturing steps.
Next, a frame is fastened or bonded to a working area of the membrane, framing the working area. After fastening, the framed working area is ready to be separated from the remaining area of the membrane.
It is this separating step that is of the utmost importance to high quality pellicle manufacture. For it is during the separating step that the framed working area must be cut away from the remainder of the membrane. Using the assembly and method of the present invention, a surprisingly successful separation is obtained.
Currently, it is known to remove the framed working area by following a two-step procedure. First, the membrane is cut outward of the frame using a suitable knife, or razor blade. Second, after cutting, a multi-component solvent formulation and/or heat is used in a conventional process to form a finished peripheral edge of the membrane.
There are contamination problems associated with the first step (i.e. cutting) and these are overcome/reduced by using a novel wet die cutter assembly and method as disclosed in U.S. patent application, Ser. No. 07/763,422, entitled "Wet Die Cutter Assembly and Method", which application is incorporated herein by reference. Essentially, the contamination problems associated with conventional knife cutting are caused by "shattering" of the membrane in the non-working area which produces contaminant particles that may collect on the frame or working area. Additional problems are caused by the membrane tearing in undesired directions and for undesired lengths causing damage to the integrity of the working area.
There are also contamination problems associated with the second step (i.e. finishing) and these are the problems that the present invention is designed to reduce/overcome. When practicing a conventional finishing step, the use of a multi-component solvent blend (or acetone) results in a contaminant film forming on the peripheral edge of the membrane. The film is left after residual solvent evaporates. The film is comprised of a combination of small to microscopic particles from each of the layers because the solvent blend is formulated to dissolve all the layers, resulting in a heterogenous mixture of solvent and polymers from each layer.
Also, the solvent seeps into and eats away at the interface between layers causing surface irregularities and resulting in further deposit of the above-identified contaminated film.
The contaminated film dries and becomes powdery over time. After pellicles are manufactured using conventional methods they are shipped in separate bags or containers. During shipping, adjacent pellicles will rub against each other which scatters the contaminant powder on the edges of the pellicles with some of it falling on the pellicle surface. Using such powdery pellicles in subsequent photolithography operations will contaminate the operation and defeat the protective purpose for using the pellicle in the first place. The presence of the film on the pellicle also makes it susceptible to microscopic shattering when handled by technicians.
Another problem is that there is unpredictability associated with the conventional finishing step. That is, pellicles formed with this step may have varying amounts of the contaminated film because the amount of such film formed from multiple-layer dissolving by the solvent will vary.
Accordingly, it is a general object of the present invention to provide a unique method for making a predictably finished edge of an optical membrane with a reduced amount of contaminant on it.
Another object of the invention is to provide a unique method for promoting contaminant-free finishing of a separated edge of a framed working area of a membrane.